1. Field of the Invention
The present invention relates generally to an overdrive unit to be mounted in an automatic transmission for vehicles, and more particularly to improving the durability of the overdrive unit.
2. Description of the Prior Art
The overdrive unit of the conventional type is shown in FIG. 1, and essentially comprises a planetary gear set which includes a revolving carrier 10 secured to an input shaft 9 which is also an output shaft for a torque converter, a planetary pinion up rotatably supported on the carrier 10, a sun or central gear 11 in mesh with the planetary pinion 14, and a ring gear 15 in mesh with the planetary pinion and connected to the output shaft 23 of the overdrive unit, and a multi-plate brake assembly 19 interposed between the sun gear 11 and the casing 16 of the overdrive unit. It is known that the planetary gear set also includes helical gears and that when torque is transmitted between the input shaft 9 and output shaft 23, a thrust is produced between the toothed surfaces in mesh of the helical gears. Therefore, the prior art overdrive unit has a thrust bearing 41 interposed between the end face of a sleeve 40 inserted between the input shaft 9 and casing 16, and one side of the sun gear 11, and a further thrust bearing 42 interposed between the rim 15A of the ring gear 15 connecting the ring gear and the output shaft and the casing 16. In the above arrangement of the prior art, the thrust bearing 41 supports any forward directed thrust exerted on the sun gear 11, and the thrust bearing 42 supports any rearwardly-directed thrust which is transmitted as indicated by an arrow A through a connections 12B between the drum 12A of the hydraulic power cylinder for the multi-plate clutch 12 and the sun gear 11, the spacer 45, the outer race 13A of the one-way clutch 13, the carrier 10, the ring gear rim 15A and the metal sheets inserted between the above-mentioned elements, and is exerted on the sun gear 11. In order to maintain the one-way clutch 13 under the optimum operating condition and provide a higher durability for the clutch, however, it is desirable to minimize any external force that may be exerted on the one-way clutch 13.